hypocotyl

thumb|Diagram of Scouler's willow (Salix scouleriana) seed, indicating position of hypocotyl. The hypocotyl (short for "hypocotyledonous stem", meaning "below seed leaf") is the stem of a germinating seedling, found below the cotyledons (seed leaves) and above the radicle (root).

==Eudicots== As the plant embryo grows at germination, it sends out a shoot called a radicle that becomes the primary root, and then penetrates down into the soil. After emergence of the radicle, the hypocotyl emerges and lifts the growing tip (usually including the seed coat) above the ground, bearing the embryonic leaves (called cotyledons), and the plumule that gives rise to the first true leaves. The hypocotyl is the primary organ of extension of the young plant and develops into the stem. The most massive hypocotyl is that of Myrmecodia lamii which can be in height by wide, and is filled with chambers and passageways used by their ant symbionts